Sketching sites using mockflow.com is pretty cool but maybe, just maybe, the best way to do it is with pencil and paper. This relies on the fact that you have decent drawing skills. If you don’t, maybe you should buy a sketch pad and start practicing. If an idea hits you, sketch it. The clients seem to like it’s simplicity and many find pencil and paper not intimidating and help the clients participate more in the most intuitive way to wireframe.
Just because there are cool digital tools, it doesn’t always mean they are the best way. I love my Kindle but sometimes holding a book and flipping through the pages is a better experience.